1. Field of the Invention
This invention relates to an apparatus for detecting the profile, coordinates of observation points, and physical amounts of a step height and a phase change of an observation object from the image information of the observation object, obtained by a differential interference microscope, and to a detection method applied to this apparatus.
2. Description of Related Art
A differential interference microscope has been widely used to observe the microscopic structure of a human body or an IC pattern because information on a phase change and a step height of an observation object can be visualized by polarization interference. Recently, various attempts have been made in particular to use differential interference microscopes for the inspection of minute projections (bumps) for intimate contact prevention of a magnetic head provided on the surface of a magnetic disk, the measurements of a defect in a phase-shift reticle used for pattern exposure of a semi-conductor and the amount of retardation (a phase difference), and the positioning device of a semiconductor wafer.
For example, each of Japanese Patent Preliminary Publications Nos. Hei 5-149719 and Hei 7-248261 discloses a technique of applying a differential interference microscope, which can be thought of as a shearing interferometer or a Mach-Zehnder interferometer, to the detection of a defect in a phase-shift reticle and a phase measurement. Japanese Patent Preliminary Publication No. Hei 7-239212 discloses a technique that the differential interference microscope is used to detect the edge of a positioning mark provided on a semi-conductor wafer, thereby positioning the wafer.
In these techniques, however, conventional interference measurement technology is merely applied to the differential interference microscope, and the influence of diffraction of light on the surface of an observation object is not taken into account. Moreover, the influence of a change intensity of light caused by a change in reflectance or transmittance of light on the observation object is not taken into account in like manner.
For these influences of the diffraction and intensity change of light on the observation object, in Japanese Patent Preliminary Publication No. Hei 9-15504, the present inventor clarifies the imaging characteristics of the differential interference microscope and provides an approach that extracts the phase information of the observation object from an image obtained by the differential interference microscope.
The differential interference microscope is such that a phase change in the surface of the observation object is converted into an image intensity distribution. Conversely, it is conceivable that, by analyzing the intensity distribution of a differential interference image, the phase change of the surface of the observation object can be detected. Further, it is set forth in Hei 7-239212 that since the edge of the step of the observation object brings about an abrupt phase change and as a result, the image intensity distribution is also abruptly changed, a portion where the image intensity distribution is abruptly changed is extracted from the differential interference image, and thereby the position of the step height of the observation object can be detected.
Japanese Patent Preliminary Publication No. Hei 5-256795 discloses a technique that uses the differential interference image of a normal sample as a reference image and compares this reference image with the image of the observation object, thereby detecting foreign matter contained in the observation object.
In the case where the phase distribution of the observation object is derived from the intensity distribution of the differential interference image, the phase change of the observation object cannot be accurately detected if factors other than the phase change of the observation object, such as changes in transmittance and reflectance of light relative to the observation object and a change in intensity of illumination light, are contained in the observation object.
This problem can be solved to some extent when the information of an object to be observed is previously acquired as the reference image to make the image processing of the observation object on the basis of data of the reference image. This method, however, requires a long time for comparison processing with the reference image, and is unfavorable in view of the fact that a reduction in time is required for the inspection of a semiconductor, for instance.
Where the edge of the step of the observation object is detected, the intensity distribution of the differential interference image in a portion which changes from a convexity to a concavity is reversed, compared with that in a portion which changes from a concavity to a convexity. Thus, in order to detect the edge, it is necessary to detect both the maxima and the minima of the intensity distribution of the differential interference image.
In this case, however, because of changes of detection characteristics of an element forming the differential interference image and intensity characteristics of illumination light, the brightness and contrast of the differential interference image are considerably changed. Moreover, there is the problem that when a gradient in a specific area on the observation object is detected, a specific value is to be detected from the differential interference image and in particular, the image becomes liable to undergo the influence of incoming disturbed light.
Where a change in the amount of phase of the step height of the observation object is measured, if the step height is relatively small, a fringe scan used for interference measurement is combined with the operation of the differential interference microscope and thereby the phase information of the observation object can be obtained. When the fringe scan is performed, four images in which the amounts of retardation between polarized components are different must be photographed for calculation, and hence a compromise to reduce the processing time cannot be effected.
When the amounts of retardation between polarized components are 0and xcfx80, images vary widely in intensity. Thus, in order to detect a correct amount of phase, an imager whose dynamic range is wide becomes necessary, which makes an apparatus complicated.
In Hei 9-15504, the present inventor devotes attention to the imaging characteristics of the differential interference microscope and provides a technique of separating phase information and intensity information from the differential interference image.
It is, therefore, an object of the present invention to provide a detection apparatus for detecting specific physical amounts of, for example, the gradient, minute planar surface, edge, step height, and phase change of an observation object, in less time than in any other conventional technique, by separating phase information and intensity information from the differential interference image of the observation object derived from a differential interference microscope, and a detection method applied to this detection apparatus.
In order to achieve this object, the detection apparatus according to the present invention includes a differential interference microscope having a light source, an illumination optical system for introducing light from the light source onto an observation object, provided with a member for splitting the light from the light source into two polarized components, and an imaging optical system for forming an image of the observation object, provided with a member for recombining the two polarized components split in the illumination optical system; a means for changing the amount of retardation between the two polarized components; a means for photographing the image of the observation object; and a means for performing a calculation on the image captured by this photographing means.
In the detection apparatus of the present invention, amounts of retardation between the two polarized components split in the illumination optical system are detected to form two differential interference images relative to the observation object in which the amounts of retardation between the polarized components are equal, but signs are different. Subsequently, in these two differential interference images, a differential calculation is performed on each set of opposite pixels to obtain a differential image, and image information in a predetermined range is extracted from the differential image. By this method, the gradient of the observation object can be detected.
Further, in the detection apparatus of the present invention, amounts of retardation between the two polarized components split in the illumination optical system are detected to form two differential interference images relative to the observation object in which the amounts of retardation between the polarized components are equal, but signs are different. Subsequently, in these two differential interference images, a differential calculation is performed on each set of opposite pixels to obtain a differential image, and the absolute value of image information on the differential image is found to set a predetermined threshold so that an image area exceeding the threshold is obtained. By this method, the edge of the observation object can be detected.
Still further, in the detection apparatus of the present invention, amounts of retardation between the two polarized components split in the illumination optical system are detected to form two differential interference images relative to the observation object in which the amounts of retardation between the polarized components are equal, but signs are different. Subsequently, in these two differential interference images, a differential calculation and a summed calculation are performed on each set of opposite pixels so that differential image information and summed image information are obtained. When xcex8 denotes the detected amount of retardation between the polarized components, D (x, y) denotes the differential image information, S (x, y) denotes the summed image information, and "PHgr" (x, y) denotes the amount of phase on the observation object corresponding to each image information, the amount of phase "PHgr" (x, y) on the surface of the observation object can be detected by detecting a differential value ∂"PHgr" (x, y)/∂r of the amount of phase on the observation object corresponding to a direction of separation r between the two polarized components to perform integral processing in the direction r, using one of the following equations:
∂"PHgr"(x, y)/∂r=kxc2x7{(1xe2x88x92cos xcex8)xc2x7D (x, y)}/{2 sin xcex8xc2x7S (x, y)}
∂"PHgr"(x, y)/∂r=kxc2x7tanxe2x88x921 [{(1xe2x88x92cos xcex8)xc2x7D (x, y)}/{2 sin xcex8xc2x7S (x, y)}]
Here, for a parameter k, in the case of a transmission observation on the observation object, k=xcex/2xcfx80, and in the case of a reflection observation on the observation object, k=xcex/4xcfx80, where xcex is the wavelength of light emitted from the light source of the detection apparatus.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.